Pethybridge and Murphy — On Phytophthora infestans. 583 



This reddisli-brown colouring matter is a very characteristic feature of 

 the oogonium ; and in addition to staining the wall of the oogonium itself, 

 it diffuses out into and stains the surrounding medium. In this way the 

 thickness of the wall becomes considerably exaggerated in appearance. 

 "When the colouring matter is removed, or at any rate is rendered less dense 

 by treatment with alkali, it is seen that the wall is in reality not very thick, 

 although it is distinctly thicker than the walls of the ordinary hyphae, and 

 its double line of contour is readily made out. (See fig. 6, Plate XLYI.) 

 Clinton describes the oogonium wall as becoming thickened by the deposition 

 on the outside of the original coat of a more or less irregular, thick, reddish- 

 brown coat. We regard this deposit as being part of the medium which 

 has become stained by diffusion. As a result of branching two oogonia, 

 and in some cases even three, are borne on one lateral hypha. (See fig. 5, 

 Plate XLVI.) 



The oogonia are distinctly brittle, and slight pressure on the cover-glass 

 siiffices to break them open and liberate the contents or the spore if present. 

 Figs. 5 and 6, Plate XLV, are from photographs showing such crushed 

 oogonia. As regards shape the majority of oogonia are pyriform, but a few 

 were observed which were almost spherical (see fig. 4, Plate XLV); and 

 all stages were observed between forms which were practically spherical and 

 those which were distinctly pear-shaped or even so much elongated as to be 

 almost club-shaped. The oosphere and oospore occupy, as a rule, only the 

 terminal swollen portion of the oogonium, although one or two cases were 

 observed where the spores themselves were somewhat pyriform. Fig. 7, 

 Plate XLVI, illustrates one of these. 



A septum is, in most cases, formed at the base of the oogonium, shutting 

 off its contents from the hypha which bears it. Sometimes, however, this 

 septum is absent, as is shown in fig. 6, Plate XLVI. 



Even in the absence of antheridia, apparently normal, thick-walled 

 oospores are produced in the oogonia, which can be discerned after treatment 

 with alkali, or by applying judicious pressure to the cover-glass. Out of 

 258 oogonia specially examined one by one for the purpose, 87, or roughly 

 one-third, contained oospores with more or less thick walls. 



The average transverse diameter of the oogonia was found to be 38^, 

 and it varied between the limits of 31;U and 46/i. These measurements agree 

 fairly closely with those given by Clinton for the oogonia which developed 

 in his cultures. 



The oogonium wall is smooth, or at the most shows some occasional 

 irregularities, and cannot be described as sculptured. Its outline is of course 



4b2 



